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 The Guts of a Relay
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CJB
Senior Modder



Lino Lakes
MN
USA

1589 Posts

Posted - 02/02/2010 :  06:45:41  Show Profile  Visit CJB's Homepage
I had to disassemble a relay at work yesterday to inspect the contacts for a QA kind of thing, so I decided to scan some images and post a little tidbit of info here to help explain the inner-workings of a relay in case there is anyone who doesn't get how they work.

This is actually a miniature SPDT relay (still rated at 30A) but the Bosch style we use here has basically the same structure.

Post labels:
30 - COM (input)
87a - NC (normally closed/connected - is on when the coil is OFF)
87 - NO (normally open/not connected - is on ONLY when the coil is ON)
85, 86 - your + and - inputs to turn on the coil, switching the output post from 87a to 87.


31.75 KB


When power and ground are applied to post 85 and 86 (from my experiences, it makes no difference which is +) it turns the coil into an electromagnet, creating a magnetic field which pulls the contact armature toward the coil.


27.85 KB

29.28 KB

That's all there is to it. These are simple little mechanisms. A semi-automatic electronic switch if you will. A SPDT (single pole [one input] double throw [two outputs]) switch that is actuated by electricity rather than by hand.

Why use it rather than a regular toggle switch you bought from radio shack? If you're trying to switch a high current load (amps), you need a high current switch. Most lighted and automotive toggle switches are rated at only a few amps. I have found heavy duty toggles rated at up to 30A before, but they're a little pricey. If you try to pull a 30A load through a switch rated at 2A, you'll end up in a melted switch and smoke (and a dead short most likely).

If you incorporate a relay, you can use the same 2A toggle switch to send low current power to the coil and let the relay do the high current switching. On most Bosch style relays, posts 30, 87 and 87a are rated to handle 30 or 40 amps.

Anyway, I hope this helps someone understand how relays work and why they can be VERY useful in our projects. If anyone has anything to add or corrections to make to my above statements, please feel free!


12V Lil Jeep; 24v Race Car Project (abandoned); Red Wrangler (WIP);18v Police Silverado
Polaris 2X ; Polaris 700; Xtreme Machine ; 12v Super 6 Firerock
Gaucho Grande; Tinkerbell Ninja; 12v S6 Trail Rider; Brute Force 24v

Mama
Senior Modder

Kansas City
Missouri
USA



2380 Posts

Posted - 02/02/2010 :  06:54:53  Show Profile
That's cool. Can you tell me how to rebuild one?
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jparthum
Senior Modder

(77627) Nederland
Texas
USA



2272 Posts

Posted - 02/02/2010 :  07:00:26  Show Profile
Great info CJB! And well explained, even I can understand it!

Do you how many 'poles' are commonly available, and do you have any supplier/brand/price point preferences?

Thanks.



(pics link to threads)

Fast, Reliable, and Cheap... Pick any two.
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CJB
Senior Modder

Lino Lakes
MN
USA



1589 Posts

Posted - 02/02/2010 :  07:12:37  Show Profile  Visit CJB's Homepage
quote:
Originally posted by jparthum

Great info CJB! And well explained, even I can understand it!

Do you how many 'poles' are commonly available, and do you have any supplier/brand/price point preferences?

Thanks.



I personally have seen single pole, double pole and triple pole, both in a single throw and double throw configuration.

Mama - I can not. Buy a new one.

12V Lil Jeep; 24v Race Car Project (abandoned); Red Wrangler (WIP);18v Police Silverado
Polaris 2X ; Polaris 700; Xtreme Machine ; 12v Super 6 Firerock
Gaucho Grande; Tinkerbell Ninja; 12v S6 Trail Rider; Brute Force 24v
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Mama
Senior Modder

Kansas City
Missouri
USA



2380 Posts

Posted - 02/02/2010 :  08:03:02  Show Profile
quote:
Originally posted by CJB

quote:
Originally posted by jparthum

Great info CJB! And well explained, even I can understand it!

Do you how many 'poles' are commonly available, and do you have any supplier/brand/price point preferences?

Thanks.



I personally have seen single pole, double pole and triple pole, both in a single throw and double throw configuration.

Mama - I can not. Buy a new one.


That's the thing.... I don't wanna buy new ones!

I have about 10 SF Vette boards with bad relays... if I could rebuild them we could get more vettes back on the road instead of parting them... then again, they are worth more in pieces.
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CJB
Senior Modder

Lino Lakes
MN
USA



1589 Posts

Posted - 02/02/2010 :  08:18:56  Show Profile  Visit CJB's Homepage
IF there is a chance you can remove the housing without breaking it (and still be able to put it back on) you could try to clean the contacts and meter out the coil. If the coil is bad, I'd say you're SOL. If the contacts are just dirty, well that could be an easy fix I suppose. Again, provided you can actually replace the housing cover.

12V Lil Jeep; 24v Race Car Project (abandoned); Red Wrangler (WIP);18v Police Silverado
Polaris 2X ; Polaris 700; Xtreme Machine ; 12v Super 6 Firerock
Gaucho Grande; Tinkerbell Ninja; 12v S6 Trail Rider; Brute Force 24v
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Mama
Senior Modder

Kansas City
Missouri
USA



2380 Posts

Posted - 02/02/2010 :  08:33:57  Show Profile
quote:
Originally posted by CJB

IF there is a chance you can remove the housing without breaking it (and still be able to put it back on) you could try to clean the contacts and meter out the coil. If the coil is bad, I'd say you're SOL. If the contacts are just dirty, well that could be an easy fix I suppose. Again, provided you can actually replace the housing cover.



I will experiment with that today and see if there is a way to do it. The board is useless right now anyway.
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carfu
New Member






24 Posts

Posted - 02/27/2010 :  20:43:35  Show Profile
Unless I am way off here, i don't think a relay can break that easily based on the excellent writeup from the OP. It is a dumb device using an electromagnet to switch to a high current circuit. Annoying buzzing from the main motor relay I am working on but it must be that it is not seeing enough current somehow. Seems borne out by the fact many anecdotes I found that buzzing relays persist even after new relay swaps. and the real culprit turned out to be a power cut causing the relay to stick and malfunction.

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jazzcat
Expert Modder

Mesa
az
USA



748 Posts

Posted - 02/28/2010 :  11:07:54  Show Profile  Visit jazzcat's Homepage
quote:
Originally posted by Mama

quote:
Originally posted by CJB

IF there is a chance you can remove the housing without breaking it (and still be able to put it back on) you could try to clean the contacts and meter out the coil. If the coil is bad, I'd say you're SOL. If the contacts are just dirty, well that could be an easy fix I suppose. Again, provided you can actually replace the housing cover.



I will experiment with that today and see if there is a way to do it. The board is useless right now anyway.


i've gotten away with cleaning contacts on relays and them working with no issues after the fact..easier to buy a new one but if you don't want to sink money into it its worth it

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oldmisterbill
New Member

Wagoner
OK
USA

29 Posts

Posted - 03/01/2010 :  10:02:52  Show Profile
If you do a search on Bosch relays you will find a lot of them for &5 to $10 depending on amperage.A side note on the double throw relaye there are 2 amp listings 1 is for the points closed by the electromagnets & 1 is for the points closed by spring return pressure.There are some 40/50 amp ones that would be great to drive the motors from.You could come up with all types of ideas with relays to switch things (making custom functions)if you think about it a bit.

Just makin life for my grandchildren as much fun as possible.
Those who hammer their guns into plows will plow for those who do not.
I am like a Cresent Wrench ,I will work anywhere , but sometimes I slip off !
Vietnam Vet & proud of it
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carfu
New Member






24 Posts

Posted - 03/02/2010 :  09:12:19  Show Profile
In my case I can confirm that the relay malfunction was entirely due to power supply issues. Swapped out the battery and resoldered some and the motor purred some kind of sweetness, clicking and buzzing in the relay completely gone.

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